Keeping Track of Elastomer

■ INJECTION MOLDING
Keeping Track of
Elastomer
Online Cross-linking. First introduced at
the K, a new processing concept integrates
extrusion, injection molding, and reactive processing in a single manufacturing
cell. The application is centered around a thermoplastic polyurethane with rubberelastic properties that is produced in an injection molding compounder by means
of online cross-linking, and is also suitable for applications at higher temperatures.
ANJA OLTMANNS
NORBERT VENNEMANN
JOCHEN MITZLER
ue to their unique properties –
high abrasion resistance, high
mechanical strength, and very good
resistance to media – thermoplastic
polyurethane elastomers (also called thermoplastic polyurethanes, TPU) have been
used in special applications for more than
40 years. Previously, however, in cases
where higher demands were placed on
temperature resistance, the application
possibilities of standard TPU were limited due to the relatively low softening point
(140°C to 180°C). Moreover, it had not
been possible to combine the elastic properties and the high recoverability with simultaneous low Shore hardness, e.g. as is
characteristic for natural rubber.
Now, the new cross-linked TPU (TPUX) Elastollan X-Flex (manufacturer: Elastogran GmbH) adds pronounced rubberelastic capabilities and good performance
at higher temperatures to the good processing and outstanding mechanical
properties of thermoplastic polyurethanes. The combination of these features, coupled with excellent bonding to
technical plastics, opens up an enormous
application potential for this material in
the automotive and mechanical engineering industries.
D
PE104206
Mues Products & Moulds GmbH (see box
on page 21) was presented on the exhibition stand of the Munich-based machine
builder KraussMaffei (Fig. 1). Thanks to
the integration of extrusion, injection
molding, and reactive processing in an injection molding compounder (IMC)
(Fig. 2, manufacturer: KraussMaffei), and
by using an innovative cross-linking stage
– the so-called X-Form process – it is possible to produce the TPU-X in a single
manufacturing cell. Cross-linking occurs
at the mold temperatures and within the
cycle times encountered in normal injection molding techniques [1].
Compared to the manufacture of conventional rubber-metal composites, the
new process presents the user with considerable advantages, because apart from
primers, the vulcanization and other subsequential stages are saved (e.g. finishing
and calibrating), plus offering a greater
freedom of design. Moreover, continuous
direct melting and the uniform processing conditions during injection molding
ensure highly consistent quality.
What’s New about TPU-X?
TPU involves copolymers that are synthesized by means of polyaddition. Hereby, the strength properties are determined
by the crystallizing hard phase that acts as
a physical network, whilst the amorphous
soft phase is responsible for flexibility and
the elastomeric properties (Fig. 3).
Reaction into polyurethane is an equilibrium reaction and is therefore reversible – which must be taken into account during thermoplastic processing. If
a certain temperature limit is exceeded,
isocyanate groups will be regenerated,
which can recombine with OH groups after processing, resulting in an increase of
molar mass (Fig. 4).
This equilibrium reaction can be utilized for cross-linking into TPU-X. Under the usual processing conditions during injection molding, the molecular
chains of the TPU are split up. The crosslinking agent added to the melt reacts
with the free ends of the molecular chains,
whereby skilful process control permits
Clear Processing Advantages
Compared to Conventional
Methods
At the K 2007, the joint development project of the partners Elastogran GmbH,
KraussMaffei Technologies GmbH, and
Translated from Kunststoffe 3/2008, pp. 38–42
18
Fig. 1. The part “Torque converter bearing“ consists of a hard (PA66 + 35% GF) and a soft component
TPU-X (pictures except (2): Elastogran)
© Carl Hanser Verlag, Munich
Kunststoffe international 3/2008
INJECTION MOLDING ■
Fig. 2. The X-Form process demonstrated on an injection molding compounder is a combination of
reactive compounding and multi-component injection molding (photo: KraussMaffei)
this process to be managed up to final
cooling of the melt.
TPU with Pronounced Rubberelastic Properties
When substituting conventional elastomer
materials (rubber) with thermoplastic
elastomers (TPE), the elastomer-specific
properties have a special relevance. Apart
from the standard testing methods such as
hardness, tensile strength, and rebound
elasticity during material selection, it is
therefore a frequent practice to apply the
compression set test (DIN 53517 or DIN
ISO 815) to assess the recovery behavior
after constant compression.
In order to examine the rubber-elastic
properties of the new TPU-X, the material was subjected to a series of tests in the
Plastics Laboratory at the Osnabrück
University of Applied Science. When
comparing the stress/elongation curves
T10 [°C]
T50 [°C]
T90 [°C]
TSSRindex
TPU (Elastollan)
57.4
94.3
150.1
0.60
TPU-X (Elastollan X-Flex)
108.3
145.2
167.0
0.83
TPO (EPDM+PP)
39.1
54.4
102.0
0.51
TPV (EPDM-X+PP)
49.0
100.7
79.2
0.61
EPDM
118.7
164.9
184.3
0.84
NR (natural rubber)
115.5
161.5
217.1
0.73
(Fig. 5) of various elastomers, the standard, non-linked TPU (Elastollan) is distinguished by a very high elongation at
break (> 1,000 %) and a relatively high
tensile strength (~30 MPa). Cross-linked
TPU-X (Elastollan X-Flex) has a somewhat lower value of 600 to 650 % for elongation at break, but this is still higher than
the typical values of carbon black-filled
elastomer materials such as NR, SBR, and
NBR. The additional cross-linking of the
new TPU-X leads to even higher strength
Fig. 3. The crystallizing hard phase determines the strength, the amorphous soft phase, flexibility,
and the elastomeric properties
Kunststoffe international 3/2008
values (above 40 MPa) than standard
TPU. Consequently, the strength of Elastollan X-Flex is superior to that of conventional elastomers and other TPE materials of comparable hardness.
A disadvantage of commercial TPE
when compared with rubber is reflected
in the starting region of the stress/elongation curve. Whilst the curves of typical
rubber materials are relatively flat in the
starting region, and only become steeper
at higher elongations, most TPEs usually
exhibit a steeper strain increase in the
starting region due to the thermoplastic
phase. Amongst others, this is one reason
why TPE often have a stiffer “feel” than
rubber materials, even though the measured hardness is the same. The enlarged
detail in Figure 5 shows clearly that in the
starting region of the stress/elongation
curve, the behavior of Elastollan X-Flex
hardly differs from that of conventional
elastomers.
Table 1. The results
of the TSSR measurement of non-linked
and cross-linked TPU
are compared with
selected TPEs and
elastomer materials
Compression Set after Constant
Deformation
Determination of the compression set is
intended to provide information about
how much of the elastic properties of
elastomers are maintained after a long
period of constant deformation. For this
purpose, tests were conducted in accordance with DIN ISO 815 at different
temperatures, as well as in accordance
with the VDA Guideline 675 216
(Method B at 100°C). Hereby, and contrary to the more frequently used
Method A, the sample is initially cooled
to room temperature in the deformed
state, and then the stress is relieved. This
procedure involves tougher testing conditions, and usually leads to poorer values than the more usual strain relief in
the warm state.
An examination of the compression set
values (Fig. 6) determined in this manner
proves that the additional cross-linking
in TPU-X clearly improves the elastic
properties when compared with standard
TPU. In spite of tougher testing conditions, the compression set values obtained V
19
■ INJECTION MOLDING
are partially lower than for other elastomer materials or TPE.
However, it is known that the results of
compression set tests are prone to considerable uncertainties, and do not always
permit sufficient differentiation between
similar materials. For this reason, two new
testing methods were recently developed,
which permit a more extensive assessment of the elastomer-specific properties
[2, 3]. Within the scope of this project,
both methods were also applied to evaluate the TPU-X samples, and will therefore be described briefly in the following.
Intermittent Strain/Elongation
Measurements
Earlier work has shown that the differences in reversibility of the deformation
behavior between TPE and rubber become particularly clear if these are examined with intermittent elongation strain
in the stepped hysteresis test using increasing deformation amplitudes [4, 5].
For these measurements, a test specimen
is deformed by tensile strain at a constant
Fig. 4. The reaction to polyurethane is an equilibrium reaction and therefore reversible – an important aspect for thermoplastic processing
elongation speed (here: 50 mm/min) up
to a defined elongation limit (here: ε1 =
20 %), and is relieved completely immediately after at the same speed.After strain
relief, the residual elongation is determined, and the procedure is repeated with
Strain/Elongation Curves
50
4
MPa
Elastollan X-Flex
Elastollan
TPV
NBR
NR
SBR
3
40
2
Strain σ
1
30
0
0
20
40
60
80 100
20
Fig. 5. Cross-linked
TPU-X is stronger
than conventional
elastomers and other
TPE materials with
comparable hardness
10
0
0
200
400
600
800
1,000 % 1,200
Elongation
© Kunststoffe
Compression Set Tests
Compression set
100
%
80
Anisothermal Test of Strain
Relaxation
Elastollan
Elastollan X-Flex
66
60
36
40
20
0
21
10
72 h/23 °C
32
16
72 h/70 °C
22 h/100 °C
© Kunststoffe
20
a simultaneous increase of the elongation
limit by the amount Δε (here: Δε = 20 %).
This sequence is repeated until the sample tears or the elongation limit reaches a
specified maximum value.
The deformation behavior’s reversibility – one of the most important elastomer-specific properties – can be assessed particularly well if the residual
elongation values determined after the individual deformation cycles are represented as a function of the elongation limit. The measurement curves (Fig. 7) show
that the behavior of SBR-based elastomer
is almost ideal, i.e. residual elongation is
low – also at high elongation limits. On
the other hand, commercial thermoplastic elastomers such as TPV (EPDM-X +
PP) or standard TPU behave differently.
Here, and with low elongation limits,
residual elongation is greater than with
rubber, and even increases disproportionately above a critical elongation.
When compared with standard grade
TPV or TPU materials, Elastollan X-Flex
exhibits a significantly better recovery behavior, which is comparable with an elastomer based on EPDM.
Fig. 6. Compared to
standard TPU, the additional cross-linking
in TPU-X improves its
elastic properties
As the physical cross-linking of TPE is
thermally reversible, the mechanical behavior under thermal stress is of major
significance. By means of the newly developed TSSR method (temperature
scanning stress relaxation), this behavior
can be analyzed in a simple manner and
with high reproducibility. Hereby, the
elastomer-specific properties in particular are reflected in the test results.
© Carl Hanser Verlag, Munich
Kunststoffe international 3/2008
INJECTION MOLDING ■
processes overcompensate the entropy
elastic behavior. Value T50 can serve as a
comparative value for the mechanicalthermal behavior of TPE and elastomers.
Extensive investigations with thermoplastic vulcanisates (TPV) have shown
that this value correlates with the value
for compression set [6], and thus represents an alternative to the time-consuming and often poorly reproducible test
method.
The TSSR index is a relative measure
for the elastomer-like temperature behavior of a TPE or elastomer. Hereby, the
theoretical behavior of an idealized elastomer material serves as reference, which
exhibits no strain reduction (strain relaxation) even at increasing temperature.
For this, the surface area below the standardized (F/F0) force/temperature curve
is determined, and put into relationship V
Progressive Loading Hysteresis Test
Residual elongation εres
140
Fig. 7. Compared to
materials such as
standard TPU or TPV,
TPU-X exhibits significantly better recovery behavior,
which is comparable
with an EPDM-based
elastomer
Elastollan X-Flex
Elastollan
SBR
TPV (EPDM-X+PP)
EPDM
%
100
80
60
40
20
0
0
50
100
150
200
250
300
%
400
Elongation ε
© Kunststoffe
maximum value for the operating temperature range, whilst T10 describes the
temperature at which strain relaxation
Standardized Force/Temperature Curves
Elastollan X-Flex
Elastollan
TPO (EPDM+PP)
Fig. 8. The TSSR
force/temperature
curves show the mechanical behavior
under simultaneous
thermal stressing of
standard TPU and
TPU-X when compared with other TPE
and elastomer materials
TPV (EPDM-X+PP)
EPDM
NR
1.2
Standardized force F/F0
The TSSR method is based on a strain
relaxation test, which is conducted under
anisothermal conditions. A detailed description of the testing method is found
in earlier papers [3, 6, 7].Within the scope
of this project, the tests were conducted
with a TSSR meter supplied by Brabender Messtechnik GmbH & Co. KG, Duisburg, Germany. Details on the equipment
are given in [8]. The following parameters can be determined from the measured force/temperature curve.
The temperature limit Tx indicates at
which temperature force F has been reduced by x %, referred to the initial force
F0. Normally, the temperature limits T10,
T50 and T90 are defined. Hereby, the value T90 must be seen as the theoretical
1.0
0.8
0.6
0.4
0.2
0
i
Project Partners
20
40
60
80
100
120
140
160
Temperature T
Materials technology:
Elastogran GmbH
Elastogranstr. 60
D-49448 Lemförde
Germany
E-Mail: [email protected]
www.elastogran.de
Plant technology:
KraussMaffei Technologies GmbH
Krauss-Maffei-Str. 2
D-80997 München
Germany
E-Mail: [email protected]
www.kraussmaffei.com
Kunststoffe international 3/2008
200
TSSR Index
1.2
Elastollan
X-Flex
1.0
TPU
0.8
TSSR Index
Mold technology:
Mues Products & Moulds GmbH
Gewerbepark Conradty 1
D-83059 Kolbermoor
Germany
www.mues-pm.de
°C
© Kunststoffe
TPV
75 A
0.4
EPDM/S
46 A
TPS
0.6
EPDM/P
EPDM/P
Elastomer
NR/S
EPDM/S
NR/S NR/P
EPDM/S
EPDM/S
EPDM/S
46 D
TPO
0.2
0
20
60
100
140
180
Temperature limit T50
220
260
°C
300
© Kunststoffe
Fig. 9. The TSSR index, represented here as a function of temperature limit T50, shows that the properties of TPU-X are similar to those of conventional elastomers of EPDM or natural rubber
21
■ INJECTION MOLDING
with the surface area of the idealized elastomer material. The greater the TSSR index value is, the more elastomer-like is the
temperature behavior of the examined
material. Extensive investigations on conventional elastomers have shown that
these come very close to the ideal behavior, and have TSSR indices of >0.8. Significantly lower values in the range of
about 0.5 to 0.7 result for TPE.
The standardized (F/F0) force/temperature curves (Fig. 8) show that the chemically cross-linked elastomer materials
based on natural rubber (NR) and EPDM
exhibit the highest temperature limits (e.g.
T50 >160°C). Contrary to this, simple
blends – like the TPO based on EPDM+PP
described here – already exhibit a pronounced strain reduction at relatively low
temperatures, so that only low T50 values
result in the range of 50 to 60°C.
In comparison, and due to dynamic
cross-linking of the elastomer phase, far
better behavior is shown by TPV, for example the EPDM-X + PP described here,
for which a significant increase of the
temperature limit T50 to values above
100 °C can be observed. Similarly good
behavior is found with TPU, including
Elastollan. Nonetheless, a relatively large
difference to conventional elastomers can
still be ascertained by all commercial
TPEs, which is also reflected in the corresponding performance characteristics.
However, comparing the curve of
TPU-X (Fig. 8) with the results determined for typical rubber samples (NR
and EPDM) reveals that the difference is
considerably less than for the other TPEs.
Similarly, a comparison of the parameters derived from the TSSR measurements
(Table 1) shows that cross-linked Elastollan X-Flex exhibits better values than the
standard TPU (Elastollan) and other TPE
materials (TPV and TPO). In terms of
these properties, it practically corresponds to conventional elastomers based
on EPDM and NR. Also the TSSR index,
a measure for a material’s compliance
with the behavior of ideal elastomers, and
shown in Figure 9 as a function of temperature limit T50, makes clear that the
properties of TPU-X are similar to those
of conventional elastomers based on
EPDM or natural rubber (NR).
Summary
The material Elastollan X-Flex represents
a novel integration of thermoplastic
polyurethane (TPU) and cross-linking
agent. It combines the good processing
properties of thermoplastic materials
22
with the rubber-elastic properties of an
elastomer. Simultaneously, the outstanding mechanical properties of TPU, and its
resistance to media and ozone are maintained.
The clearly improved behavior of
cross-linked TPU at higher temperatures
opens up a range of new applications that
were previously impossible with standard
TPU. Due to the combination of these
properties, TPU-X is a highly interesting
alternative to the rubber compounds used
so far – not only in the automotive industry. ■
ACKNOWLEDGEMENT
Special thanks are due to Markus Seidl, Mues Products & Moulds GmbH, for the realization of this project.
REFERENCES
1 Mitzler, J.; Hilmer, K.; Seidl, M.: A Simple Alternative to Rubber-Metal Composites. Kunststoffe
International (10/2007) 10, pp. 126–130
2 Vennemann, N.; Hündorf, J.; Kummerlöwe, C.;
Schulz, P.: Phasenmorphologie und Relaxationsverhalten von SEBS/PP-Blends. Kautsch. Gummi
Kunstst. 54 (2001), pp. 362–367
3 Vennemann, N.: Praxisgerechte Prüfung von TPE.
Kautsch. Gummi Kunstst. 55 (2003), pp. 242–249
4 Eisele, U.: Spezifische Merkmale von Gummi im
Vergleich zu anderen Werkstoffen. Kautsch. Gummi Kunstst., Sonderdruck Celle (1987), pp. 17–22
5 Vennemann, N.; Leifheit, S.; Schulz, P.: TPE Test
for Automotive Engineering. Kunststoffe plast
europe 90 (2000) 8, pp. 46–48
6 Reid, C.G.; Cai, K.G.; Tran, H.; Vennemann, N.: Polyolefin TPV for Automotive Interior Applications.
Kautsch. Gummi Kunstst. 57 (2004), pp. 227–234
7 Barbe, A.; Bökamp, K.; Kummerlöwe, C.; Sollmann, H.; Vennemann, N.; Vinzelberg, S.: Investigation of Modified SEBS-Based Thermoplastic
Elastomers by Temperature Scanning Stress
Relaxation Measurements. Polymer Engineering &
Science 45 (2005), pp. 1498–1507
8 Fuchs, F.: Grenzen aufzeigen – anisotherme Spannungsrelaxionsmessung. Kautsch. Gummi Kunstst.
59 (2006), pp. 302–303
THE AUTHORS
DIPL.-ING. ANJA OLTMANNS, born in 1966, works
for Elastogran GmbH, Lemförde, Germany, in Sales
and Technical Service of the Automotive European
Business Unit Injection Molding Thermoplastic
Polyurethanes.
PROF. DR. NORBERT VENNEMANN, born in 1953,
is head of the Plastics Laboratory in the Faculty of
Engineering and Computer Science at the Osnabrück
University of Applied Science;
contact: www.ecs.fh-osnabrück.de
DIPL.-ING. (FH) JOCHEN MITZLER, born in 1973, is
head of Product and Technology Management at
KraussMaffei Technologies GmbH, Munich, Germany.
© Carl Hanser Verlag, Munich
Kunststoffe international 3/2008